Method of hydrophobing silica

ABSTRACT

A method of producing hydrophobic colloidal silica is disclosed. The method comprises combining an aqueous colloidal silica dispersion, polyorganosiloxane, and anionic surfactant to yield a stable aqueous emulsion of hydrophobic colloidal silica. The emulsion may be further processed by evaporating the water to yield a white material which may be easily ground to a fine powder of hydrophobic colloidal silica. This powder is useful as a reinforcement for silicone rubber.

This is a continuation-in-part of co-pending application Ser. No.094,970, filed on Sept. 10, 1987, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for producing hydrophobic colloidalsilica.

2. Description of the Prior Art

Iler in U.S. Pat. No. 2,786,042, issued Mar. 19, 1957, described aprocess of adding an aqueous solution of a silanolate which is a strongalkali salt of a monovalent hydrocarbon-substituted silanol to anaqueous sol of colloidal particles of amorphous silica. These treatedsilica particles could be recovered by drying or they would betransferred from the original aquasol to an organosol by mixing theaquasol after treatment at a pH below 6 with an organic liquid andremoving the water.

Another method of producing hydrophobic silicas was shown by Wetzel inU.S. Pat. No. 2.802.850, issued Aug. 13, 1957, wherein a loweralkyltrichlorosilane was hydrolysed in a water-in-oil emulsioncontaining an alkali-metal silicate. The treated silica was filteredfrom the oil, washed with water, and dried to yield the desired product.The treated silica is suitable for the preparation of greases and asfillers for elastomeric materials.

Youngs in U.S. Pat. No. 3,634,288, issued Jan. 11, 1972, prepared ahydrophobic silica sol by emulsifying a silica hydrosol in an oildispersing medium, stripping water from the system, adding a siliconeoil, and heating to allow the silicone oil to react with the silica -OHgroups, resulting in an oil dispersion of the hydrophobic silica.

These processes yielded a dried, treated silica particle that washydrophobic, or an organosol of the treated silica. The process of theinstant invention provides a storage stable water emulsion ofhydrophobic colloidal silica. This emulsion may be dried to yield acrusty, white material which may be easily ground to a fine fluffy whitepowder. The treated powder is hydrophobic and oleophilic.

Johnson et al. teach in U.S. Pat. No. 4,221,688, issued Sept. 9, 1980, asilicone emulsion which provides an elastomeric product. Their emulsionconsists essentially of an anionically stabilized hydroxylatedpolydiorganosiloxane which contains about 2 silicon-bonded hydroxyls permolecule, an organic tin compound, and a colloidal silica, the emulsionhaving a pH in the range of 9 to 11.5 inclusive.

SUMMARY OF THE INVENTION

This invention comprises a method for making colloidal silica that ishydrophobic. The method consists essentially of combining an aqueouscolloidal silica dispersion, and a hydroxyl containing, low molecularweight polyorganosiloxane in the form of an emulsion produced using ananionic surfactant such as dodecylbenzene sulfonic acid. After theemulsion mixture is homogeneous, its pH is adjusted to a range of 7.5 to10 to provide a storage stable aqueous emulsion of hydrophobic colloidalsilica.

The aqueous emulsion of hydrophobic colloidal silica may be dried toprovide a solid hydrophobic silica which may be comminuted to form afine powder which is not dispersable or soluble in water but isdispersable in toluene.

The powdered hydrophobic colloidal silica may be used as a reinforcementfor silicone rubber. The aqueous emulsion of hydrophobic colloidalsilica may be mixed with an emulsion of polydiorganosiloxane in emulsionform to yield a reinforced polymer when the water is removed from theemulsion.

DESCRIPTION OF THE INVENTION

This invention relates to a method for making colloidal silicahydrophobic consisting essentially of (I) combining to form an aqueousemulsion, (a) an anionic surfactant, (b) aqueous colloidal silica havingprimary silica particle diameters in the range of 2 to 100 nanometresand silica concentrations up to 65 percent based on the combined weightof silica and water, and (c) polyorganosiloxane having a average of from3 to 40 siloxane RR'SiO and an average of from 0 to 10 siloxane units ofthe R_(4-x) Si where x has an average value greater than 2 with amaximum of 4 and each unsatisfied valence of silica in the R_(4-x) Sibeing satisfied with a hydroxyl radical or a divalent oxygen atom bondedto silicon atoms, the polyorganosiloxane being terminated by at leasttwo hydroxyl radicals and the hydroxyl content not exceeding 15 weightpercent based on the weight of the polyorganosiloxane, each R and R' inthe polyorganosiloxane being selected from a monovalent radical of thegroup consisting of lower alkyl, aryl, alkenyl, aralkyl, and haloalkyl,the polyorganosiloxane being a liquid polymer or copolymer at processingtemperature and having a weight average molecular weight of less than4000; the ratio of silica to polyorganosiloxane being at least 2:1; and(II) adjusting the pH of the emulsion into the range of 7.0 to 10 toprovide a storage stable aqueous emulsion of hydrophobic colloidalsilica.

Colloidal sized particles of silica have been found useful as thickenersand as reinforcing filler in silicone rubber. When silicone rubber isreinforced with untreated silica, a reaction takes place that causes themixture to become tough and nervy, making it difficult to furtherprocess the mixture unless such processing is done immediately after themixture is made. It is known that such a reaction, known as structuring,or crepe aging, can be prevented by treating the surface of the silicawith materials to react with the hydroxyl radicals present on the silicasurface. Many methods have been devised for treating silica as a powderand as a water dispersion to prevent this undesired structuring. Theinstant invention is a simplified method of producing a treated silicaparticularly useful for reinforcing silicone rubber.

The colloidal silica used in the instant invention is commerciallyavailable. These aqueous silica sols, known as hydrosols, aredispersions of silica in water, having a pH of from about 8.2 to about11.0, stabilized with sodium ions, aluminum ions, or ammonia. Theprimary silica particles can vary from 2 to 100 nanometres in averagediameter with a preferable range of from 5 to 25 nanometres. Thehydrosols can have up to 65 percent by weight silica with the usualrange varying from 15 to 50 percent silica by weight.

The polyorganosiloxane useful in the instant invention is a lowmolecular weight hydroxyl containing polymer or copolymer which isliquid at processing temperature. The maximum weight average molecularweight of the polyorganosiloxane is less than 4000. Thepolyorganosiloxane has an average of from 3 to 40 siloxane units of theformula RR'SiO and an average of from 0 to 10 siloxane units of theformula R_(4-x) Si, the polyorganosiloxane being terminated by at leasttwo hydroxyl radicals but the hydroxyl content not exceeding 15 percentbased on the weight of the polyorganosiloxane. Each R and R' in thepolyorganosiloxane are selected from a monovalent radical of the groupconsisting of lower alkyl, aryl, alkenyl, aralkyl, and haloalkyl. Thepolyorganosiloxane has R and R' selected such that thepolyorganosiloxane is a liquid at processing temperature. If all of theorganic radicals were phenyl for example, the polymer could be anintractable, solid crystalline material at processing temperature.Preferably, R and R' are methyl, ethyl, phenyl, vinyl, and3,3,3,tri-fluoropropyl radicals with methyl being most preferred becauseit is readily available. The value of x has an average value of greaterthan 2 with a maximum of 4. Each unsatisfied valence of silica in theformula R_(4-x) Si is satisfied with a hydroxyl radical or a divalentoxygen atom bonded to a silicon atom. The preferred polyorganosiloxaneis a hydroxyl endblocked polydimethylsiloxane having about 4 weightpercent hydroxyl radical with a molecular weight in the range of 750 to800.

An anionic surfactant is necessary to emulsify the polydiorganosiloxane.Anionic surfactants which are useful include salts of surface activesulfonic acids, alkali metal sulforicinates, sulfonated glycerylestersof fatty acids, salts of sulfonated monovalent alcohol esters, amides ofamino sulfonic acid such as the sodium salt of oleyl methyl-tauride,sulfonated aromatic hydrocarbon salts such as sodium alpha-naphtholenemonosulphate, condensation products of naphthelene sulfonic acids withformaldehyde, and sulfates such as ammonium lauryl sulfate, triethanolamine lauryl sulfate, and sodium lauryl ether sulfate. The preferredanionic surfactants are the alkali metal salts of the sulfonic acids,particularly the sodium salts. The sulfonic acid can be illustrated byaliphatically substituted benzenesulfonic acids, aliphaticallysubstituted naphthelene sulphonic acids, aliphatic sulfonic acids,silylalkylsulfonic acids, and aliphatically substituted diphenylethersulfonic acids. Particularly useful anionic surfactants aredodecylbenzenesulfonic acid and the sodium salt ofdodecylbenzenesulfonic acid.

The combining of the anionic surfactant, colloidal silica, andpolyorganosiloxane can be performed in several ways. One useful methodconsists of adding the polyorganosiloxane to an aqueous colloidal silicadispersion along with the anionic surfactant, then stirring to obtain ahomogeneous mixed emulsion. The emulsion is then adjusted as with a 1percent by weight hydrochloric acid solution to a pH above 7. Theadjusted emulsion is storage stable. The hydrophobic silica may be usedin the emulsified form or it may be dried by evaporating the water fromit to yield a crusty material which is easily ground to a fine whitepowder which will not dissolve or disperse in water but is easilydispersed in toluene.

A second useful method of combining the anionic surfactant, colloidalsilica, and polyorganosiloxane involves the preparation of an emulsionof the polyorganosiloxane. The polyorganosiloxane, water, and anionicsurfactant are mixed together, then homogenized by passing through acolloid mill or similar intense-type mixer. Three passes through acolloid mill at 316 kg per cm² gave a stable emulsion. In this case, thesodium salt of dodecylbenzene sulfonic acid was used as the anionicsurfactant. The strongly basic colloidal silica dispersion is mixed withanionic surfactant and the pH is adjusted with dilute hydrochloric acidto a pH of just above 7. The emulsion of polyorganosiloxane is thenslowly stirred into the colloidal silica emulsion. The homogeneousmixture then will yield a hydrophobic silica in the same manner as thefirst method.

A modified procedure may be used to yield a hydrophobic silica withimproved reinforcing properties in silicone rubber by structuring acolloidal silica dispersion before treatment. An aqueous colloidalsilica dispersion is stirred with a cation exchange resin to lower thepH to the acid side. Optimum results are obtained with a pH of about 5.The acidified colloidal silica is then aged while stirring. Thestructuring of the silica raises the viscosity of the dispersion andraises the pH. After the colloidal silica dispersion has structured, itis slowly added with stirring to a polyorganosiloxane emulsion preparedas discussed above in the second method. After mixing to give ahomogeneous mixture, the pH is raised to between 7 and 10 with dilutesodium hydroxide and stirring continued. After a period, such as 1 hour,the viscosity will drop to yield an emulsion of hydrophobic colloidalsilica similar to that prepared by the other methods except, when usedto reinforce silicone rubber, this method will yield higher physicalproperties in the compounded and cured silicone rubber.

The amount of polyorganosiloxane used in the method of the instantinvention is dependent upon the amount of colloidal silica in theemulsion. Useful products are obtained when the amount ofpolyorganosiloxane is below 50 percent by weight of the weight ofcolloidal silica present. When used as reinforcement in silicone rubber,the useful amount is between 5 and 35 percent. Below 6 percent, theamount of polyorganosiloxane is not sufficient to fully react with thesilica surface so that when such a material is used to reinforcesilicone rubber, the reinforced silicone rubber stock will become toughand nervy on storage. When above 35 percent of polyorganosiloxane isused, the hydrophobic colloidal silica, when mixed into silicone rubber,will yield a reinforced silicone rubber stock that will be sticky on thesurface.

The amount of anionic surfactant used is based upon the amount ofpolyorganosiloxane used. There must be sufficient anionic surfactantpresent to emulsify the polyorganosiloxane used. When using the sodiumsalt of dodecylbenzene sulfonic acid, about 1 percent of the surfactantbased on the weight of polyorganosiloxane is sufficient to emulsify thepolyorganosiloxane. Higher levels of about 3 percent were found to yieldhigher physical properties in a silicone rubber compounded using thehydrophobic colloidal silica produced in this method. Amounts ofsurfactant in excess would adversely effect the properties of thefinished articles produced using such a hydrophobic colloidal silica.The amount of surfactant based on the total solids in the emulsion canvary from 0.2 to 2 percent. Particularly useful emulsions are producedat about 1 percent surfactant based upon total solids in the emulsion.

The method of the instant invention yields a stable aqueous emulsion ofhydrophobic colloidal silica when the pH of the final emulsion isbetween 7 and 10. This stable emulsion can be used in conjunction withemulsified silicone polymers to produce reinforced silicone basessuitable for compounding into silicone rubber stocks. The polymeremulsion and silica emulsion are mixed together, then allowed to age fora period of time, for instance from 1 to 4 days, to allow for anyinteraction between them. The mixed emulsion is then dried byevaporating the water by air drying and vacuum drying at elevatedtemperature to yield a dry reinforced silicone rubber base. The base isthen compounded on a 2 roll mill with organic peroxide catalyst and anyother desirable silicone rubber additives in the conventional,well-known manner.

The stable hydrophobic colloidal silica emulsion of the instantinvention may also be used by evaporating the water from the emulsion byair drying or heating to yield a crusty, white material. This materialis easily ground to a fine white powder. This powder is a hydrophobicsilica which is suitable for the conventional uses of such materials. Itmay be used as reinforcement in conventional silicone rubber gum toproduce a reinforced silicone rubber base that is not subject to thestructuring on aging that is observed when untreated colloidal silica ismixed with a silicone rubber gum.

The following examples are presented for purposes of illustrating theinvention and should not be construed as limiting the scope of theinvention which is properly delineated in the claims.

EXAMPLE 1

This example illustrates one method of producing a hydrophobic silica.

A mixture of 200 g of a colloidal silica having 30 weight percent ofSiO₂, pH of 9.7, and a particle size of 70 to 80 Angstrom and 20 g of ashortchain hydroxyl endblocked polydimethylsiloxane fluid having 3.9 gweight percent hydroxyl radical was rapidly stirred for 16 hours at 23°C. The mixture was given 3 passes at 316 km/cm² through a homogenizer.No mixing or emulsification resulted; on standing the mixture separatedinto two phases. Then 0.704 g of dodecylbenzenesulfonic acid (DBSA) in 5g of water (≅1% by weight DBSA on solids) were added with simplestirring to achieve a homogeneous mixture. The emulsion was neutralizedto a pH of 7.3 using 79 g of 1 percent by volume hydrochloric acid.

This emulsion remained stable for over 80 days.

When a portion of the above emulsion was dried, a crusty white powderremained which was easily ground to a fine white powder. This powderwould not dissolve or disperse in water and greatly lowered the surfacetension of the water. The powder was easily dispersed in toluene.

EXAMPLE 2

A second method of producing a hydrophobic silica is shown.

A mixture of 60 g of the polydimethylsiloxane fluid of Example 1, 138 gof water, and 2.65 g of a 23% solution of a sodium salt ofdodecylbenzenesulfonic acid was homogenized at 316 kg/cm² for 3 passsesto give a stable mechanical emulsion (2.99% solids).

A mixture of 600 g of the colloidal silica of Example 1 and 1.8 g ofdodecylbenzenesulfonic were mixed together, stirred well, and adjustedto a pH of 7.7 using 106 ml of 1.2 molar hydrochloric acid.

While stirring the silica emulsion, the polydimethylsiloxane emulsionwas slowly added, then stirred for 1 hour. This mixed emulsion appearedsimilar to that of Example 1.

EXAMPLE 3

A third method of producing a hydrophobic silica is shown. This silicawas modified to produce a more highly structured silica before beingmade hydrophobic.

The colloidal silica of Example 1 was treated with sufficient ionexchange resin to reduce the pH to 4.8. The silica was stirred in aclosed system for 20 hours. Then 58 g of this aged acidic silica wasadded with stirring to 19.3 g of the polydimethylsiloxane emulsion ofExample 2. After mixing, the pH was adjusted to 8.0 with a 2 percent byweight solution of sodium hydroxide. The silica emulsion was thick as itwas added, causing the mixture of silica and polydimethylsiloxane to bethick. After stirring for an hour at the pH of 8, the mixture dropped inviscosity to a lower viscosity. The final emulsion was stable for atleast 49 days.

When dried, the emulsion left a crusty white material which was easilyground to a fluffy white powder which floated on water but dispersedreadily in toluene.

EXAMPLE 4

A series of compositions were prepared illustrating one method of usingthe hydrophobic silica of this invention.

A high molecular weight silicone polymer was prepared in an emulsion. Amixture of 1122 g of cyclodimethylsiloxanes having from 3 to 8dimethylsiloxy units per molecule, 1.96 g of methylvinylcyclosiloxane,1800 g of water, and 34.7 g of dodecylbenzenesulfonic acid werehomogenized by passing 3 times through a homogenizer at 316 kg/cm². Theemulsion was heated for 3 hours at 90° C. while stirring, then cooled to2°-4° C. for about 4 days followed by adjusting to a pH of 7.5 usingabout 240 ml of a 2 percent solution of sodium hydroxide. The emulsionwas 31.7% solids. A portion of the emulsion was coagulated and thepolymer recovered. The polymer had a weight average molecular weight of576,000 and a Williams Plasticity of 0.96 mm.

(A) A portion of this polymer emulsion was blended with untreatedcolloidal silica.

A mixture of 200 g of this polymer emulsion (63.4 g of polymer) wasmixed with 63.3 g of the untreated colloidal silica of Example 1 (19.0 gof silica). This resulted in 30 parts of silica per 100 parts ofpolymer. After 24 hours at room temperature, the pH was adjusted from9.9 to 7.5 by adding a 1 percent solution of acetic acid. The emulsionwas poured into a large Teflon lined pan and placed in a hood for 48hours to evaporate the water. The product was 84 g of an extremelybrittle, waxy material. When it was attempted to mill a portion of thematerial, it could not be done; it merely ground to a finer powder.

(B) A portion of this polymer emulsion was blended with treatedcolloidal silica.

A mixture of 437 g of this polymer emulsion and 225 g of the colloidalsilica emulsion of Example 1 after treating were shaken together andallowed to stand at room temperature for 4 days. This mixture was thencoagulated by pouring in a dish, air drying at room temperature andvacuum oven drying at 80°-100° C. A 60 g portion of the driedpolymer-silica mixture was milled together with 0.48 g of 2-5 bis(tertbutylperoxy)-2,5-dimethylhexane catalyst at 50% active material toyield a catalysed material. This was pressed into a sheet and cured 10minutes at 171° C. The cured sheet was cut into specified test bars andtested according to ASTM D 2240 for durometer and ASTM D 412 for tensilestrength and ultimate elongation. The results were as shown in Table I.

(C) A portion of the polymer emulsion was cured without the silicareinforcement.

The material and procedure of 4B was repeated, but the colloidal silicaemulsion was not added to the polymer emulsion.

The results were as shown in Table I.

(D) A portion of the polymer emulsion was blended with the structuredhydrophobic silica.

A mixture of 170 g of this polymer emulsion and 72 g of the structuredhydrophobic silica emulsion of Example 3 were stirred together for 24hours at room temperature. The mixture was coagulated by pouring in adish, air drying at room temperature and vacuum oven drying at 100° C.for 2 hours. The dried mixture was catalyzed, molded, and tested as inExample 4B. The results were as shown in Table I.

                  TABLE I                                                         ______________________________________                                                              Tensile                                                 Sample  Durometer     Strength Elongation                                     ______________________________________                                        4A      no usable     MPA      Percent                                                product                                                               4B      22            1.96     770                                            4C      11            0.21     177                                            4D      26            5.12     743                                            ______________________________________                                    

This example illustrates the usefulness of the treated silica inreinforcing a silicone polymer.

That which is claimed is
 1. A method for making colloidal silicahydrophobic consisting essentially of(I) combining to form an aqueousemulsion,(a) an anionic surfactant, (b) aqueous colloidal silica havingprimary silica particle diameters in the range of 2 to 100 nanometresand silica concentrations up to 65 percent based on the combined weightof silica and water, and (c) polyorganosiloxane having an average offrom 3 to 40 siloxane units of the formula RR'SiO and an average of from0 to 10 siloxane units of the formula R_(4-x) Si where x has an averagevalue greater than 2 with a maximum of 4 and each unsatisfied valence ofsilica in the formula R_(4-x) Si being satisfied with a hydroxyl radicalor a divalent oxygen atom bonded to silicon atoms, thepolyorganosiloxane being terminated by at least two hydroxyl radicalsand the hydroxyl content not exceeding 15 weight percent based on theweight of the polyorganosiloxane, each R and R' in thepolyorganosiloxane being selected from a monvalent radical of the groupconsisting of lower alkyl, aryl, alkenyl, aralky, and haloalkyl; theratio of silica to polyorganosiloxane being at least 2:1 and (II)adjusting the pH of the emulsion into the range of 7.0 to 10 to providea storage stable aqueous emulsion of hydrophobic colloidal silica. 2.The method of claim 1 in which the anionic surfactant is selected fromthe group consisting of a surface active sulfonic acid and a sodium saltof a surface active sulfonic acid, and the polyorganosiloxane isessentially a hydroxyl endblocked polydiorganosiloxane.
 3. The method ofclaim 2 in which the anionic surfactant is present in an amount of from0.2 percent to 2 percent by weight based on the weight of total solidspresent and the polydiorganosiloxane is present in an amount of from 6percent to 35 percent by weight based on the weight of colloidal silicapresent.
 4. The method of claim 3 in which the anionic surfactant isselected from the group consisting of alkyl benzenesulfonic acid, sodiumsalt of alkyl benzenesulfonic acid, and alkyl hydrogen sulfate and thepolydiorganosiloxane is a hydroxyl endblocked polydiorganosiloxane inwhich the organo groups are selected from the group consisting ofmethyl, ethyl, vinyl, phenyl, and 3,3,3,trifluoropropyl radicals, atleast 50 percent of the radicals being methyl radicals.
 5. The method inaccordance with claim 1 in which the water is evaporated from theemulsion to provide a solid hydrophobic silica which is comminuted toform a fine powder which is not dispersable or soluble in water but isdispersable in toluene.
 6. A method for making colloidal silicahydrophobic consisting essentially of(I) combining to form an aqueousemulsion,(a) an anionic surfactant, (b) aqueous colloidal silica havingprimary silica particle diameters in the range of 2 to 100 nanometresand silica concentrations up to 65 percent based on the combined weightof silica and water, the aqueous colloidal silica having first beenstructured by aging at pH of 4 to 5 before combining with (a) and (c),and (c) polyorganosiloxane having an average of from 3 to 40 siloxaneunits of the formula RR'SiO and an average of from 0 to 10 siloxaneunits of the formula R_(4-x) Si where x has an average value greaterthan 2 with a maximum of 4 and each unsatisfied valence of silica in theformula R_(4-x) Si being satisfied with a hydroxyl radical or a divalentoxygen atom bonded to silicon atoms, the polyorganosiloxane beingterminated by at least two hydroxyl radicals and the hydroxyl contentnot exceeding 15 weight percent based on the weight of thepolyorganosiloxane, each R and R' in the polyorganosiloxane beingselected from a monovalent radical of the group consisting of loweralkyl, aryl, alkenyl, aralkyl, and haloalkyl; the ratio of silica topolyorganosiloxane being at least 2:1 and (II) adjusting the pH of theemulsion into the range of 7.0 to 10to provide a storage stable aqueousemulsion of hydrophobic colloidal silica.
 7. The method of claim 6 inwhich the anionic surfactant is selected from the group consisting of asurface active sulfonic acid and a sodium salt of a surface activesulfonic acid, and the polyorganosiloxane is essentially a hydroxylendblocked polydiorganosiloxane.
 8. The method of claim 7 in which theanionic surfactant is present in an amount of from 0.2 percent to 2percent by weight based on the weight of total solids present and thepolydiorganosiloxane is present in an amount of from 6 percent to 35percent by weight based on the weight of colloidal silica present. 9.The method of claim 8 in which the anionic surfactant is selected fromthe group consisting of alkyl benzenesulfonic acid, sodium salt of alkylbenzenesulfonic acid, and alkyl hydrogen sulfate and thepolydiorganosiloxane is a hydroxyl endblocked polydiorganosiloxane inwhich the organo groups are selected from the group consisting ofmethyl, ethyl, vinyl, phenyl, and 3,3,3,trifluoropropyl radicals, atleast 50 percent of the radicals being methyl radicals.
 10. The methodin accordance with claim 6 in which the water is evaporated from theemulsion to provide a solid hydrophobic silica which is comminuted toform a fine powder which is not dispersable or soluble in water but isdispersible in toluene.
 11. A method for making colloidal silicahydrophobic consisting essentially of(I) combining to form an aqueousemulsion,(a) an anionic surfactant, (b) aqueous colloidal silica havingprimary silica particle diameters in the range of 2 to 100 nanometresand silica concentrations up to 65 percent based on the combined weightof silica and water, and (c) polyorganosiloxane having an average offrom 3 to 40 siloxane units of the formula RR'SiO and an average of from0 to 10 siloxane units of the formula R_(4-x) Si where x has an averagevalue greater than 2 with a maximum of 4 and each unsatisfied valence ofsilica in the formula R_(4-x) Si being satisfied with a hydroxyl radicalor a divalent oxygen atom bonded to silicon atoms, thepolyorganosiloxane being terminated by at least two hydroxyl radicalsand the hydroxyl content not exceeding 15 weight percent based on theweight of the polyorganosiloxane, each R and R' in thepolyorganosiloxane being selected from a monovalent radical of the groupconsisting of lower alkyl, aryl, alkenyl, aralkyl, and haloalkyl; theratio of silica to polyorganosiloxane being at least 2:1 and (II)adjusting the pH of the emulsion into the range of 7.0 to 10 to providea storage stable aqueous emulsion of hydrophobic colloidal silica, then(III) evaporating the water from the emulsion to provide a solidhydrophobic silica which is comminuted to form a fine powder which isnot dispersable or soluble in water but is dispersable in toluene.
 12. Amethod for making colloidal silica hydrophobic consisting essentiallyof(I) combining to form an aqueous emulsion,(a) from 0.2 to 2 percent byweight based upon the weight of the total solids present of an anionicsurfactant selected from the group consisting of a surface activesulfonic acid and a sodium salt of a surface active sulfonic acid, (b)aqueous colloidal silica having primary silica particle diameters in therange of 2 to 100 nanometres and silica concentrations up to 65 percentbased on the combined weight of silica and water, and (c) from 6 to 35percent by weight based on the weight of colloidal silica ofpolyorganosiloxane having an average of from 3 to 40 siloxane units ofthe formula RR'SiO, the polyorganosiloxane being terminated by at leasttwo hydroxyl radicals and the hydroxyl content not exceeding 15 weightpercent based on the weight of the polyorganosiloxane, each R and R' inthe polyorganosiloxane being selected from a monovalent radical of thegroup consisting of lower alkyl, aryl, alkenyl, aralkyl, and haloalkyl,and (II) adjusting the pH of the emulsion into the range of 7.0 to 10 toprovide a storage stable aqueous emulsion of hydrophobic colloidalsilica.
 13. The method of claim 12 in which the anionic surfactant isselected from the group consisting of alkyl benzenesulfonic acid, sodiumsalt of alkyl benzenesulfonic acid, and alkyl hydrogen sulfate and thepolydiorganosiloxane is a hydroxyl endblocked polydiorganosiloxane inwhich the organo groups are selected from the group consisting ofmethyl, ethyl, vinyl, phenyl, and 3,3,3,trifluoropropyl radicals, atleast 50 percent of the radicals being methyl radicals.